Defects in the innate immune response are implicated in autoimmune diseases, including Systemic Lupus Erythematosus (SLE). During activation of the innate immune response, immunological tolerance is maintained while polyclonal B cell activation promotes immunity to the invading pathogen. However, in autoimmune-prone individuals polyclonal activators induced by viral infection overcome B cell tolerance. Recently, we described that soluble mediators secreted by dendritic cells (DCs) and macrophages (MOs) maintain tolerance during activation of the innate immune system. IL-6 and sCD40L selectively repress immunoglobulin (Ig) secretion by autoreactive B cells yet had no repressive effect on naive B cells. In this application we propose to define the molecular basis for the selective repression of Ig secretion by DCs and MOD, and to test the hypothesis that defects in DC/Mo-mediated tolerance contribute to the autoimmune phenotype of lupus-prone mice. The finding that IL-6R ligation selectively represses LPS-induced Ig secretion by autoreactive B cells suggests that chronic BCR stimulation reprograms the IL-6R, thereby regulating innate immune responses. In aim 1, we propose to define the mechanism(s) whereby IL-6 differentially regulates LPS-induced Ig secretion in autoreactive, compared to naive B cells. The findings that multiple repressive factors regulated Ig secretion suggested that IL-6 and sCD40L could exhibit redundant, or possibly non-overlapping functions. In subaim 2a, we will identify the B cell subset(s) repressed by DCs (IL-6) and MOs (IL-6+sCD40L), and determine whether dyregulation of DC/MO-mediated tolerance facilitates autoantibody secretion by select B cell subsets within autoimmune mice. Apoptotic cells are implicated in SLE and may activate autoreactive B cells. In subaim 2b, we propose to assess whether apoptotic cells diminish secretion of IL-6 and sCD40L by DC and MO-IL-6, thereby indirectly activating autoreactive B cells. In addition, we will determine whether apoptotic cells disrupt IL-6R reprogramming in B cells allowing autoreactive B cells to become activated. DCs and MO from lupus-prone mice are defective in repressing autoantibody production in vitro, although it remains unclear whether these defects cause the autoimmune phenotype in vivo. In subaim 3a, we propose to generate mixed bone marrow chimeras to address if reconstitution of MRL/lpr mice by wildtype DCs and MOs prevents the onset of autoimmunity or restores tolerance. To investigate if the loss of IL-6 and CD40L induce autoimmunity in non-autoimmune mice, we propose in aim 3b to monitor autoantibody production in IL-6"'" x CD40L"'" mice.